"Mandy, luv," his voice was incredibly soothing, as her his hands on her body, holding her close to still stroking, hoping to rid her of some of this tension. "It's not your fault. You were fourteen. You were trying to help. It's not your fault."
He cupped her face in his large hands, gently tipping her head up til she would look at him. "It was no one's fault except for the person that cast that killing curse." it wasn't the easiest thing for him to say, as it also made him directly responsible for his actions.
He pressed a kiss to her lips, infinitely gentle. "Should haves, would haves, could haves, you can't dwell on those. You were a kid, and even if you should have listened, you couldn't be expected to in that situation. It was tragic, but it was an accident, and it wasn't your fault."
He was sure she wouldn't believe him, but he spoke the truth here. "Killing him, Mandy," he said slowly, "would involve you actually casting the curse or actively doing him harm. Trust me, I know," he said heavily but with purpose. If she knew about his past, he could at least try to get her to see the truth of her own situation.